The World Health Organization (WHO) has classified obesity as a worldwide pandemic, resulting in about 300,000 deaths per year in U.S. adults and $80 billion in expenses (1-11). Obesity is a major cause of morbidity and mortality within our VA medical system accounting for the majority of cases of diabetes mellitus, hypertension, coronary artery disease and cerebrovascular accidents. The current understanding of the regulation of eating behavior and obesity suggests that both central and peripheral receptors and pathways control appetite. High protein diets have been described to reduce appetite, but the mechanisms are unknown. Our research program has been focused on the role of the recently discovered neuropeptide, Pituitary Adenylate Cyclase Activating Polypeptide (PACAP) and its high affinity receptor, PAC1, in the regulation of appetite and in the development of obesity. PACAP is released in the hypothalamus, where it acts with a dual mechanism: it activates the anorexigenic alpha- melanocortin pathway and it stimulates the release of the orexigenic PYY neuropeptide. The peripheral pathway by which PACAP regulates appetite has not been previously explored. Previously, we have shown that PACAP is released from the gastric mucosa cells in response to meal stimulation and that neuroendocrine of the stomach express PAC1 to regulate gastrointestinal physiological function. For this proposal, we have developed a PAC1 deficient mouse model (PAC1-/-) and have demonstrated that these mice develop hyperphagia and obesity and thus serve as a useful model to investigate the regulatory pathways mediating obesity. Based on these preliminary data, our central hypothesis is that PACAP, released in the stomach following protein ingestion, stimulates PAC1 expressed on gastric neuroendocrine cells to inhibit ghrelin release, thereby suppressing appetite. Accordingly, in Aim 1 of the proposal we will study the relationship between dietary protein content, PACAP release and effects on the downstream pathways connecting PAC1 and ghrelin release. To accomplish this we propose to utilize our PAC1 deficient mice and assess the response to protein dietary content, body mass and feeding behavior. The feeding behavior will be analyzed using our recently acquired BIODAQ equipment, EchoMRI to measure mouse fat and lean mass. The serum levels of PACAP, ghrelin and of a panel of obesity associated hormones and cytokines will be measured using Luminex. The second specific aim of the proposal will investigate the in vitro cellular responses to luminal protein using our recently developed cellular isolation techniques. Microarray and RT-PCR studies will be used to analyze genetic profiles of isolated ECL and X/A. A clear understanding of the key pathophysiological mechanisms underlying these PACAP- induced regulatory pathways involved in appetite/satiety regulation, will allow the design of future clinical studies with a focus on therapy. The proposed studies will elucidate the peripheral mechanisms, by which PACAP influences protein-induced satiety which will provide an important therapeutic target for the development of novel therapeutic compounds which would be used to treat obesity in our Veterans and active duty military personnel.